When trying to unsubscribe from mail notifications, I was not able to remove my subscription from the database.

quick look in the parameters passed from the browser shows the following:

user[user_mail_notifications_attributes][0][id]=2
user[user_mail_notifications_attributes][0][interval]=#<OpenStruct to_s="", to_translation="No emails">
user[user_mail_notifications_attributes][0][mail_notification_id]=1